Apparatuses for visually displaying an electrical video signal are well known in the art. A notable example is the cathode ray tube (CRT). The CRT is a raster scan device in which sequential electrical signals are received by the CRT and are displayed sequentially on a screen. The sequential displaying of the electrical signals is so fast, coupled with the persistance of the phosphor on the CRT, that an image which is suitable for viewing is created on the screen. Although the CRT has found wide acceptance in displaying television type electrical signals, its noteworthy shortcoming is that it cannot be made economically to provide a large viewing area. In particular, for large viewing area such as 100 cm.times.75 cm, the CRT must be so large and weigh so much in order to withstand the impact of an implosion, that the CRT cannot be employed in such application.
The principle upon which the CRT operates, i.e., cathodoluminescence, whereby electrons are emitted and are accelerated to a phosphor screen impinging thereon to create the visual display, is particularly well suited for visually displaying television type signals A cathodoluminescent display provides high resolution as well as brightness of the image on the screen. Thus, for large area screen viewing it is desirable to use the principle of cathodoluminescence.
U.S Pat. No. 4,076,994 discloses a type of display tube based on cathodoluminescence that is suitable for large screen application. The apparatus disclosed therein, however, requires an evacuated envelope containing a source to generate electrons and to direct them in paths and to guide them and to deflect them toward a phosphor screen. The structure required to direct, guide, and confine the electrons within an evacuated envelope appears to complicate the apparatus.
Another approach which has been used for large area viewing is by projecting the image on a large screen. However, such technique suffers from the disadvantages of a lack of sharpness in the image and of low intensity or low brightness of the image. In addition, in some cases, the brightness of the image is also dependent upon the angle of viewing.
Image tubes are well known in the art. An image tube is an electron device that reproduces on its fluorescent screen an image of the radiation pattern focused on its photosensitive surface. These tubes are used when it is desired to have an output image that is brighter than the input image or to convert non-visible radiation from an image into a visible display. Typically, however, most image tubes have diameter sizes of 18, 25 and 40 mm and do not have the capability of displaying a color image.
One type of an image tube is called the "proximity focus" tube, in which the photocathode and the phosphor screen are in parallel planes spaced closely together. In this type of apparatus an entire image is directed at a photocathode, releasing photoelectrons. The photoelectrons are directed at the phosphor screen; they are not focused but because of the high field between the phosphor screen and the photocathode, they do not deviate much from trajectories that are parallel to the tube's axis. However, the tube is small and the screen image erect, but the resolution is not as good as that for other types of image tubes employing focusing means. (See RCA Electro-Optics Hand Book, 1974 page 173-180.) U.S. Pat. No. 4,208,577 discloses a "proximity type" amplifier tube for amplifying an x-ray image.
An image tube employing a plurality of stages to magnify the image, using focusing means such as magnetic or electrostatic field, is also well known.